Vaporization is the transition from a liquid to a gas or vapor. For example, when heat energy is added to water, it reaches its boiling point and becomes steam. The amount of energy needed to vaporize a given mass of liquid at a constant temperature is called the heat of vaporization. Vaporization can also occur at the surface of a liquid through evaporation. Outdoors, the kinetic energy is often contributed by the sun and wind. If the energy added exceeds that of the intermolecular forces, molecules can become vapor. Evaporation of sweat is an important way for many animals to cool off. Sweat is mostly water. It can absorb heat from the skin, increasing the kinetic energy of the water molecules and causing it to evaporate from the skin's surface. This results in a cooling effect which helps maintain body temperature. In plants, evaporation is essential during transpiration. Water evaporating from the surface of leaves helps draw up neighboring molecules, with the help of cohesion and adhesion, bringing water from the roots to the leaves.